[OPENMP]Use new interface for task reduction.

Summary:
Patch forces codegen to use the new runtime functions for task reductions where
the issue with passing the address of the original variables to the UDR
initializers is fixed. Also, this patch is required for upcoming
support of task modifier inreduction clause.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78733